The invention relates to a process and arrangement for measuring a physical quantity by means of an optical sensor which impresses a power modulation on an effective light flux I.sub.1 in a monotonic correlation with changes of the measured quantity.
German Published Unexamined Patent Application (DE-OS) No. 2,903,821 discloses the provision of encoding a measured quantity using a ratio of light intensities of two part light fluxes, one of which, the effective light flux, is subjected to an intensity modulation proportional to the measured quantity, while the other, the reference light flux, remains unaffected by the measured quantity, the two part light fluxes being generated by division of a primary light flux emitted from a single light source.
The intensity modulation of the effective light flux takes place, for example, by means of a diaphragm in the form of a wedge gap which can be displaced back and forth transversely to the effective light flux in relation to the measured quantity. Depending on whether the measured quantity increases or decreases, this diaphragm occludes more or less of the cross-sectional area of the beam.
For the case where the measured quantity is a temperature, a bimetal strip, exposed to this temperature, may be used to displace the diaphragm proportional to the measured quantity wherein one end of the bimetal strip executes deflection movements proportional to the temperature. Wherever the part light fluxes to be subjected to the intensity comparison are guided to remotely arranged photodetectors via an individual optical fiber, the light fluxes are also exposed hereby, i.e. part from the intensity modulation of the effective light flux proportional to the measured quantity, generally, to different factors which influence the light intensity and impair the accuracy of the measurement. Such significant influences include changes in the transmission paths caused by, for example, variations in reflectively occurring at ends of the optical fiber and/or on optical elements for directing the light fluxes into the respective optical fiber and for directing light fluxes onto the respective detector. Such variations, therefore, can influence the measuring result.
In order to avoid measuring errors resulting from such differences in transmission paths, German Published Unexamined Patent Application (DE-OS) No. 2,903,821 discloses guiding the effective light flux and the reference light flux in each case via the same fiber-optic transmission path, so that these influences act in a same way on the light fluxes to be compared with each other and are at the same time eliminated during ratioing.
In order to separate the two light fluxes from each other in a simple way, it is then necessary to generate them in different spectral distribution. Spectral images suitable for this are known from the technique of optical telecommunication in wavelength division multiplexing mode (Hans F. Mahlein, Elektronik 13, 1983, pages 80-86).
However, it is disadvantageous in this case that, due to their dispersion characteristics for light fluxes of different wavelength, optical fibers also have a different attenuation, which in turn impairs the measuring accuracy.
To avoid the disadvantage explained thus far, the process disclosed by German Published Unexamined Patent Application (DE-OS) No. 3,528,294 is suitable.
According to the process of (DE-OS) No. 3,528,294, the physical quantity to be measured is determined from a ratio of the luminous powers of an effective light flux I.sub.1 and a reference light flux I.sub.2, the light fluxes I.sub.1, and I.sub.2 being guided via the same optical fibers from a light source to a sensor imparting a power modulation to at least the effective light flux I.sub.1. The fluxes I.sub.1 and I.sub.2 are guided by common optic fibers to a detector and evaluation unit, in which a power measurement and evaluation are carried out in units of the measured quantity. Both the effective light flux I.sub.1 and the reference light flux I.sub.2 each have a comb-shaped spectral distribution with a plurality of components in the form of spectral lines, the effective light flux I.sub.1 and the reference light flux I.sub.2 being nested one in the other in spectral terms, so that the two light fluxes have the same or approximately the same mean optical frequency. This is intended to ensure that the two light fluxes are exposed to the same influences by the transmission path, irrespective of the power modulation, and to that extent a path-neutral transmission of the light fluxes imparting the spectral coding of the measured quantity becomes possible. As a result, the influences of optical transmission and coupling elements present in the measurement path act in the same way on the two light fluxes and therefore cancel each other out.
However, a disadvantage of this process is that interference filters of complicated design are required both in the sensor and in the detector unit because of the plurality of the spectral components of the effective light flux and of the reference light flux. These filters have to be tuned exactly in terms of their transmission properties to identical spectral distributions, thus involving a high technical outlay. The filter arrangements, which can be positioned very far away from one another in terms of space, are very easily disturbed, for example when the sensor filter arrangement is exposed to different ambient conditions, for example, a higher temperature than the detector filter arrangement, with the adverse result that the filters are out of tune with one another and the accuracy of the measurement is considerably impaired.
Accordingly an object of the present invention is to provide an improved process of the type mentioned in the introduction, to the effect that the technical outlay necessary for carrying out the process is appreciably reduced and, at the same time, a reduction of the influence of disturbance variables on the measurement result is also obtained.
A further object of the present invention is to provide arrangements suitable for carrying out the process of the invention.
In accordance with the process of the present invention a spectral coordination of the effective light flux I.sub.1 and the reference light flux I.sub.2 is achieved such that the spectral distribution S(.lambda.) of the effective light flux I.sub.1 and the spectral distribution R(.lambda.) of the reference light flux I.sub.2 have the same effective wavelength .lambda.z. As a result, a very good path neutrality of the transmission of the optical signals in a fiber-optical transmission path is obtained, specifically even when their transmission varies linearly with the wavelength, this usually being true in a good approximation.
This will be demonstrated in detail by the following consideration:
Starting from a constant transmission EQU T.sub.1 (.lambda.)=c.sub.1
of the fiber-optical transmission path and a sensor setting linked to a predetermined value of the measured quantity x, with an associated spectral distribution S(.lambda.,x) of the effective light flux and spectral distribution R(.lambda.,x) of the reference light flux, the intensity values I.sub.S1 and I.sub.R1 in a signal or effective-light detection channel are obtained according to the relations EQU I.sub.S1 =C.sub.1 .intg.S(.lambda.,x)d.lambda. (1)
and EQU I.sub.R1 =C.sub.1 .intg.R(.lambda.,x)d.lambda.. (2)
For a transmission linearly dependent on the wavelength .lambda., according to the relation EQU T.sub.2 (.lambda.)=C.sub.2 (1+.alpha..lambda.) (3)
for the intensity I.sub.S2 in the effective-light detection channel there is the relation EQU I.sub.S2 =C.sub.2 .intg.(1+.alpha..lambda.).multidot.S(.lambda.,x)d.lambda.(4)
and for the intensity I.sub.R2 of the reference-light flux there is the relation EQU I.sub.R2 =C.sub.2 .intg.(1+.alpha..lambda.).multidot.R(.lambda.,x)d.lambda.. (5)
Path neutrality is obtained when the following is true: EQU I.sub.S1 /I.sub.R1 =I.sub.S2 /I.sub.R2 ( 6)
By substituting the relations (1), (2) and (4), (5) in the relation (6), after a simple conversion the following relation is obtained: EQU .intg..lambda..multidot.R(.lambda.,x)d.lambda./.intg.R(.lambda.,x)d.lambda. =.intg..lambda..multidot.S(.lambda.)d.lambda./.intg.S(.lambda.,x)d.lambda.( 7)
this being equivalent to stating that the centers of gravity .lambda..sub.z of the original signal and reference spectrum must coincide.
However, this relationship (7) can be complied with in a very close approximation by means of filters of simple design, for example, by means of interference filters which can be of substantially simpler construction than the filter arrangements necessary in the use of the process disclosed by (DE-OS) No. 3,528,294 and which are therefore also less sensitive to varying ambient influences.
Such an arrangement with interference filters used both as spectral filters and as beam splitters or beam unifiers contemplated by one embodiment of the present invention.
In an embodiment of the arrangement for carrying out the process according to the invention, the separation of the signal light flux, utilized for the intensity comparison, from the reference light flux is possible electronically, so that, in principle, a single spectral filter provided within the frame of the sensor device is sufficient.
Alternatively to a neutral wedge provided as a sensor element according to one embodiment of the arrangement and shiftable in relation to the measured quantity, a spectral wedge for the power modulation of the effective light flux can be used especially advantageously, with the result that an especially simple overall construction of the arrangement can be obtained. By means of an additional absorption filter which, in the reference light flux, largely suppresses the spectral components utilized for the effective light flux, improved dynamics with the effect of an increased range of variation of the ratio I.sub.1 /I.sub.2 are achieved, thus making an especially good signal-to-noise ratio possible.
Diffraction elements with a locally variable diffraction coefficient can also be provided as movable, sensor elements.
In an arrangement according to a further embodiment it is possible in a simple way to adjust the spectral center of gravity of the reference light flux in a controlled way and/or keep it at a specific wavelength .lambda..sub.z by means of a control which is obtained in a simple way.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.